1. Field of the Invention
The present invention relates to an image processing device capable of decoding a high-quality, high-resolution color image from a small amount of image data.
2. Description of the Related Art
In order to output a fine image in a color printer, it is necessary to simultaneously satisfy a fine gray scale and high resolution. However, a large amount of information is necessary to improve the gray scale and the resolution, so that a required memory capacity increases. When it is assumed that information of eight bits per color of RGB is provided, information of 24 bits per pixel is necessary. In the case where the resolution is 600 dpi, the data amount of an image in A4 size can reach 96 Mbytes. When a large amount of data is used, the following big problems arise. The cost of a memory for holding the data is high. Moreover, it takes long time to transfer and process data, and drawing time is long.
In order to solve the problems, as disclosed in, for example, Japanese Published Unexamined Patent Application No. 11-296670, a technique of generating a high-resolution image from a low-resolution image of a fine gray scale by an interpolating process is used. In the method, however, a pixel is generated by an interpolating process, so that the same picture quality as that of an image generated at high resolution from the beginning cannot be obtained. Especially in a character image, degradation in picture quality such as a distortion peculiar to the interpolating process is conspicuous.
In another technique, while maintaining the resolution, information of colors of which number is smaller than the number of pixels per block of a predetermined size is held in each block, and a value indicative of the assigned color is assigned in each of the pixels. According to the method, for example, since each of the pixels does not have to hold full-color information, the data amount can be reduced. In the method, however, if the size of each block is not large, the data amount cannot be reduced so much. When the size of the block is increased, however, the pixels in the block have to be expressed only by the limited number of colors. It therefore causes a problem such that the color reproducibility becomes low and the picture quality is degraded.